In measurement and automation technology, the viscosity of a fluid flowing in a pipe, particularly of a liquid, is frequently determined by means of meters which, using a rotary transducer or a vibratory transducer and meter electronics connected thereto, induce internal friction forces in the fluid and derive therefrom a viscosity signal corresponding to the respective viscosity.
Such viscometers and methods of measuring viscosity by means of such meters are described, for example, in U.S. Pat. Nos. 4,524,610, 4,704,898, 4,754,640, 4,920,787, 4,922,745, 5,157,962, 5,228,331, 5,448,921, in WO-A 95/16897, EP-A 527 176 or in EP-A 1 158 289 as well as in European Patent Applications 01 120343.7 and 01 121 869.0, which were not published prior to the filing date of the present application. For generating friction forces acting in the fluid, for example, shear forces, each of these viscometers shown therein comprise an electro-mechanical transducer with a flow tube for conducting the fluid.
In general the in-line viscosimeters as mentioned above could be classified into two groups, firstly viscometers comprising a vibrating or rotating excitation element, for example, a vibrating fork or a rotating cylinder, and secondly viscosmeters with a vibrating flow tube. Latter are described, for example, in U.S. Pat. No. 4,524,610, WO-A 95/16897 or in EP-A 1158289 as well as in the above referred European Patent Applications 01 120343.7 and 01 121 869.0, wherein the viscometers shown therein comprise a vibratory transducer with a flow tube for conducting the fluid, said flow tube communicating with the pipe and vibrating in operation. Each of said vibratory transducers has an excitation assembly for vibrating the flow tube, and a sensor arrangement for sensing vibrations of the flow tube and for generating at least one sensor signal representing the vibrations of the flow tube. Further, the viscometers comprise meter electronics which deliver an excitation current for the excitation assembly and at least one viscosity value representing the instantaneous viscosity of the fluid.
Viscometers of the above type are power-line-operated devices and must therefore have at least two electric leads, i.e., two wires. The viscosity signal, which is preferably proportional to the viscosity of the fluid, is produced and output according to a standard established for this purpose, such as the 4- to 20-mA current standard and/or a usual frequency standard and/or a digital standard; for this, at least two further wires are necessary.